1. Field of the Invention
The present invention relates to a process for recovering a high-purity inert gas by purifying a raw gas containing at least carbonaceous impurities and an inert gas in a high concentration and to an apparatus therefor.
2. Description of the Prior Art
Inert gas is frequently used as a protective gas or the like in, for example, metal refinement, hot treatment, welding, and the electronic industry. Recently, inert gas has been used as an ambient atmosphere in furnaces for producing semiconductor single crystals. An inert gas which is used in the production of semiconductor single crystals must have a very high purity in order to obtain high-purity semi-conductor single crystals.
An inert gas used as a protective gas or ambient atmosphere gas is inevitably contaminated with a large quantity of impurities according to its condition of use, so that the used inert gas can not be reused as such. In general, therefore, the used gas is exhausted (discharged) into the air.
Because the used inert gas contains a large quantity of precious inert gas besides a large amount of impurities, it is not economical to discharge the gas as such into the air. It is therefore resonable that the waste gas having a high concentration of an inert gas is used as a raw gas to recover a high-purity inert gas by removing the impurities from the raw gas.
Known methods for recovering such an inert gas include one disclosed in Japanese Patent Laid-Open No. 72394/1977.
This method comprises adding oxygen to a raw gas having a high inert gas (e.g., argon) concentration, leading the raw gas containing the oxygen to a first reaction tower packed with a metallic catalyst such as palladium or platinum, where the combustible component in the raw gas is reacted with oxygen; adding hydrogen to the gas leaving the first reaction tower; leading the raw gas containing hydrogen to a second reaction tower packed with a similar catalyst as above, wherein the oxygen in the raw gas is reacted with the added hydrogen; passing this raw gas through an adsorption tower to remove carbon dioxide and water in the gas by adsorption; and passing the gas leaving the adsorption tower through a low-temperature liquefaction/separation apparatus to separate and recover a high-purity inert gas (argon).
Conventional well-known processes can remove argon effectively without any problems when the carbonaceous combustible component content of a raw gas used in recovering an inert gas (e.g., waste gas having a high argon concentration) is low. However these processes have the problem that hydrocarbons remain in the recovered argon when the content of the carbonaceous combustible component in the raw gas is increased.
The problems encountered in the prior art will now be described specifically. Semiconductor single crystal producing furnaces where argon gas is used include those of an atmospheric pressure type in which argon gas is fed to a furnace at atmospheric pressure and those of a reduced pressure type in which argon gas is fed to a furnace evacuated to a vacuum or a reduced pressure, among which the latter are becoming predominant. Especially in case of a furnace of a reduced pressure type, a hydraulic rotary vacuum pump is used for pressure reduction and the raw gas discharged from such a pump contains a large quantity of a carbonaceous combustible component. Namely, in case of a reduced pressure type of furnace where a vacuum pump is used, the waste gas contains hydrocarbons, such as CH.sub.4, in high concentrations as impurities, in addition to inorganic gases consisting mainly of N.sub.2 and O.sub.2. At the exit of the vacuum pump, the gas contains oil mist-containing heavy hydrocarbons in high concentrations in addition to light hydrocarbons.
An example of an analysis of the composition of a waste gas from a reduced pressure furnace is set forth below:
Analysis of the composition of a waste gas:
inorganic gases (N.sub.2, O.sub.2, etc.)=2 mol % PA1 light hydrocarbons (CH.sub.4 -C.sub.5)=12,000 ppm (in terms of CH.sub.4) PA1 heavy hydrocarbons (C.sub.6 -)=20,000 ppm (in terms of CH.sub.4) PA1 argon gas (Ar)=the balance PA1 CO.sub.2 concentration; 1.2% PA1 H.sub.2 concentration; 3.4% PA1 O.sub.2 concentration; 1-1.4% PA1 reaction temperature; 250.degree.-350.degree. C. PA1 CO.sub.2 concentration; 1.2% PA1 H.sub.2 concentration; 0.6-1.4% PA1 O.sub.2 concentration; .ltoreq.1 ppm PA1 CH.sub.4 concentration; 5-50 ppm
If oxygen is added to this gas (raw gas) and the combustible component is reacted with oxygen in a first reaction tower packed with a metallic catalyst, the temperature at the exit of the first reaction tower becomes extraordinary high. Further, the concentration of carbon dioxide (CO.sub.2) formed from the reaction is increased. After the addition of hydrogen, the raw gas leaving the first reaction tower is led to a second reaction tower where O.sub.2 and H.sub.2 in the raw gas are allowed to react with each other. The O.sub.2 can be converted into H.sub.2 O by this reaction. However, because the raw gas leaving the first reaction tower contains a large quantity of CO.sub.2 as described above, the following reactions, which can be neglected when the concentration of CO.sub.2 in the raw gas is low, occur: EQU CO.sub.2 +H.sub.2 .revreaction.CO+H.sub.2 O EQU CO+3 H.sub.2 .revreaction.CH.sub.4 +H.sub.2 O.
Namely, a difficulty is brought about in that the CO.sub.2 produced by the reaction in the first reaction tower reacts again with the hydrogen in the second reaction tower to produce hydrocarbons. The hydrocarbons produced in the second reaction tower are difficult to separate by adsorption, or remove by separation in the downstream adsorption tower or the low-temperature liquefaction/separation apparatus. Therefore, the hydrocarbons, which constitute part of the carbonaceous combustible component, remain in the recovered argon.
An example of an experiment will be set forth below:
composition of a gas at the inlet of the second reaction tower:
composition of a gas at the exit of the second reaction tower:
Since the tolerable hydrocarbon concentration in argon gas used in a single crystal producing furance must be extremely low (e.g., 1 ppm or below), the argon, which is recovered with much effort, can not be used without further treatment.
Namely, the prior art is effective only when the content of a combustible component is low, and has a problem that impurities remain in the recovered inert gas (Ar in the above case) when the content of a combustible component is increased.